Concrete Pier Calculator
Calculate concrete volume for deck and fence pier footings from diameter and depth. Enter values for instant results with step-by-step formulas.
Formula
V = Pi x r^2 x h (per pier); Total = V x number of piers x waste factor
Where r = radius of the pier in feet (diameter/2/12), h = depth in feet (depth/12). Volume is calculated for a cylinder, then multiplied by the number of piers and a 15% waste factor to account for over-excavation and spillage.
Worked Examples
Example 1: Standard Deck Footings - 8 Piers
Problem: Calculate concrete needed for 8 piers, each 12 inches in diameter and 48 inches deep. Concrete costs $145/cu yd.
Solution: Radius = 6 in = 0.5 ft\nDepth = 48 in = 4 ft\nVolume per pier = Pi x 0.5^2 x 4 = 3.14 cu ft\nTotal volume = 3.14 x 8 = 25.13 cu ft = 0.93 cu yd\nWith 15% waste = 1.07 cu yd\n80-lb bags = ceil(25.13 x 1.15 / 0.6) = 49 bags\nReady-mix cost: 1.07 x $145 = $155\nBag cost (80 lb): 49 x $6.50 = $319
Result: Volume: 1.07 cu yd | 49 bags (80 lb) | Ready-mix: $155 | Bags: $319
Example 2: Large Pier Footings for Covered Porch
Problem: Calculate concrete for 6 piers at 18-inch diameter and 60 inches deep for a covered porch with heavy roofing.
Solution: Radius = 9 in = 0.75 ft\nDepth = 60 in = 5 ft\nVolume per pier = Pi x 0.75^2 x 5 = 8.84 cu ft\nTotal = 8.84 x 6 = 53.01 cu ft = 1.96 cu yd\nWith 15% waste = 2.26 cu yd\n80-lb bags = ceil(53.01 x 1.15 / 0.6) = 102 bags\nReady-mix cost: 2.26 x $145 = $328
Result: Volume: 2.26 cu yd | 102 bags (80 lb) | Ready-mix: $328 | Weight per pier: 1,326 lbs
Frequently Asked Questions
What diameter should concrete pier footings be?
The required diameter depends on the load being supported and the bearing capacity of the soil at your site. For standard residential deck posts supporting typical loads, 12-inch diameter piers are the most common minimum requirement in most building codes. Larger structures, heavier loads, or weak soil conditions may require 16-inch, 18-inch, or even 24-inch diameter piers to spread the load over a larger bearing area. As a general rule, the bearing area of the pier doubles when you increase the diameter from 12 to 16 inches because area follows the square of the radius. Always check your local building code or consult a structural engineer, especially for load-bearing structures like elevated decks or room additions.
How deep should pier footings be?
Pier footings must extend below the frost line in your region to prevent frost heave, which can lift and shift the structure above. In the southern United States, the frost line may be only 12 to 18 inches deep, while in northern states and Canada it can range from 36 to 60 inches or more. Your local building code specifies the exact frost depth for your area, and the bottom of the pier must be at or below that depth. Beyond frost protection, the pier should also reach undisturbed native soil or compacted fill, which typically requires a minimum depth of 24 inches even in frost-free areas. Adding 6 inches of gravel at the bottom of the hole before pouring concrete improves drainage and prevents water from pooling under the footing.
Should I use ready-mix or bagged concrete for piers?
Bagged concrete is more practical for small projects with fewer than 10 piers because you can mix it on demand without scheduling a delivery truck or meeting a minimum order requirement. Ready-mix concrete from a truck becomes more cost-effective when the total volume exceeds 1 to 2 cubic yards, which typically corresponds to about 12 or more 12-inch diameter piers at 4 feet deep. Ready-mix also provides a more consistent mix quality and allows you to pour all piers in a single session, which is important for structural consistency. The tradeoff is that ready-mix trucks often charge short-load fees for orders under 3 to 5 cubic yards, and you need to have all your forms and rebar ready before the truck arrives. For most typical deck footing projects with 4 to 8 piers, bagged concrete is the practical and economical choice.
Do I need a Sonotube for pier footings?
Sonotubes (cardboard form tubes) are strongly recommended for pier footings because they create a smooth, uniform cylindrical shape that maximizes the structural performance of the concrete column. Without a form tube, concrete poured directly into a drilled hole takes on the irregular shape of the surrounding soil, which wastes concrete and can create weak points where soil pockets leave voids. Sonotubes also extend above grade level, allowing you to pour the pier higher than the ground surface to keep the wood post above moisture and potential termite entry points. The cost of Sonotubes is relatively modest at $5 to $15 per linear foot depending on diameter, making them an inexpensive investment in the quality and longevity of the foundation. Some building inspectors require Sonotubes or equivalent forming for code compliance.
How do I calculate concrete volume for a round pier?
The volume of a cylindrical pier is calculated using the formula V = Pi times radius squared times height, where all dimensions are in the same units. Convert the diameter from inches to feet by dividing by 12, then divide by 2 to get the radius in feet. For example, a 12-inch diameter pier that is 48 inches deep has a radius of 0.5 feet and a depth of 4 feet, giving a volume of 3.14159 times 0.25 times 4 equals 3.14 cubic feet per pier. To convert cubic feet to cubic yards for ordering ready-mix concrete, divide the total cubic feet by 27. Always add a waste factor of 10 to 15 percent to your calculated volume to account for spillage, over-excavation of holes, and variations in hole diameter.
How many bags of concrete do I need per pier?
The number of bags depends on the pier diameter, depth, and bag size. An 80-pound bag of concrete yields approximately 0.6 cubic feet when mixed, while a 60-pound bag yields about 0.45 cubic feet. For a standard 12-inch diameter pier that is 48 inches deep, the volume is approximately 3.14 cubic feet, requiring about 6 bags of 80-pound mix or 7 bags of 60-pound mix. A larger 16-inch diameter pier at the same depth requires about 5.58 cubic feet, needing approximately 10 bags of 80-pound mix. Always round up to the next whole bag and add 10 to 15 percent extra for waste, irregular holes, and the gravel drainage layer at the bottom. Buying one or two extra bags is better than running short mid-pour and creating a weak cold joint in the pier.